(a) Fields of the Invention
The present invention relates to semiconductor devices made of a group-III nitride semiconductor using a Schottky junction therein, such as Schottky barrier diodes.
(b) Description of Related Art
Group-III nitride semiconductors typified by AlxGa1-xN (where x is 0≦x≦1) have outstanding physical properties of high breakdown field and high saturated electron velocity, which exceed those of silicon (Si) or gallium arsenide (GaAs). Owing to their advantages, power devices using group-III nitride semiconductors, such as field effect transistors (abbreviated hereinafter as “FETs”) or Schottky barrier diodes (abbreviated hereinafter as “SBDs”), have been developed actively.
For example, as SBDs, two types of their conventional examples that will be shown below are known. Among them, a large number of lateral SBDs, like the first conventional example disclosed in Japanese Unexamined Patent Publication No. 2004-31896, are reported which use two-dimensional electron gas (abbreviated hereinafter as “2DEG”) generated from a heterojunction to pass a current in the direction parallel to a substrate for the purpose of providing a high forward current. Although the lateral SBD can provide a high forward current by utilizing the 2DEG effect, it also has a problem of large reverse leakage current. To solve this problem, as the second conventional example, a vertical SBD designed to pass a current in the perpendicular direction to a substrate is reported in Japanese Unexamined Patent Publication No. 2003-60212.
The second conventional example will now be described with reference to FIG. 12. Referring to FIG. 12, a buffer layer 102, a drift layer 103, an insulating film 104, and a Schottky electrode 105 are sequentially formed on a substrate 101 of silicon. The buffer layer 102 is made of gallium nitride (GaN). The drift layer 103 is made of n−-type GaN. The insulating film 104 has an opening. The Schottky electrode 105 extends through the opening of the insulating film 104 to come into contact with the drift layer 103. An ohmic electrode 106 is formed on a surface of the substrate 101 opposite to the buffer layer 102. As shown above, by fabricating the SBD to have a vertical structure with no 2DEG, a reverse leakage current can be decreased as compared with the lateral SBD.
The vertical SBD according to the second conventional example, however, has the following problem. The breakdown voltage of the vertical SBD is determined by the carrier concentration and thickness of the drift layer 103. However, during epitaxial growth of the nitride semiconductor constituting the drift layer 103, the background concentration of impurities is typically as high as about a second half of 1016 cm−3, so that it is difficult to perform doping at a lower carrier concentration less than that concentration. Thus, the breakdown voltage of the vertical SBD predominantly depends on the background carrier concentration, and thereby the SBD cannot have a high breakdown voltage.